The invention concerns impregnated textile web materials, so-called prepregs, suitable for the manufacture of honeycomb sandwich composites. In particular, the invention concerns prepregs that are impregnated with resins of dicyanates or polycyanates, multi-functional aromatic alcohols as well as suitable rheologic modifying agents and are suitable as cover layers (for example, in the form of fiberglass prepregs) for sandwich components for airplane interiors produced by the crushed-core method.
For panels used in airplane interiors, lightweight sandwich components with honeycomb materials are predominantly used. The exposed areas visible to the passengers, for example, the window trim, should have excellent surface quality. At the same time, these components must have excellent flame resistance. The requirements posed in civil aviation (interior paneling) in regard to fire behavior include minimal combustibility, minimal heat release rate, low smoke gas density as well as minimal toxicity of the fumes generated in a fire.
For this purpose, the honeycomb materials are coated at least on one side with a resin in the form of a semi-finished part, a so-called prepreg, and then compression-molded. The prepreg is an impregnated textile web material, for example, a fabric, woven material or nonwoven material of a suitable fiber material such as glass or the like, that is impregnated with a resin suitable for the described purpose.
In particular because of the above mentioned high fire behavior requirements, up to now primarily phenolic resins have been used as resins for the aforementioned purposes. However, phenolic resins cannot provide the required mechanical properties. For applications where the components are subjected to impact loads, for example, in the overhead luggage compartments, the high brittleness of the phenolic resins is often a problem. Moreover, polycondensation of the phenolic resins or the gases that are released during curing are most likely also the reason that the surface quality of the sandwich components produced today (based on phenolic resin prepregs) is not satisfactory so that these components must be manually post-processed (by spackling/filling and grinding) in a labor-intensive way. This process is time-consuming and cost intensive. Therefore, it is desirable to develop prepreg resins that produce sandwich components that have excellent surfaces already after completion of the so-called crushed-core method so that manual post-treatment steps are obsolete. The crushed-core method is a method according to which substantially plane or only slightly curved sandwich panels with strongly curved (tapering) edge areas are produced in such a way that the honeycomb core structure during the compression-molding (and curing) step is compression molded together with its prepreg cover layers in the selected mold and thus deformed and partially also compressed; however, the strength and the stiffness in these deformed and partially destroyed areas does not significantly decrease as a result of the deformation and partial destruction.
Depending on the type of application, additional properties such as excellent impact behavior are required, for example, in the case of freight compartments or overhead luggage compartments.
Moreover, when producing sandwich components by means of the crushed-core method, there are requirements in regard to the adhesive behavior (so-called tack or reactivation of tack) taking into account the shelf life (stability during storage) and handling of the prepregs; this must be ensured by modification (formulation) of the resin.
In order to formulate prepreg resins that provide honeycomb sandwich components with excellent surfaces, the use of addition resins appears to be more promising than the use of condensation resins because during curing no gases will be released. Addition resins with excellent mechanical properties are epoxide resins and cyanate resins. However, the epoxide resins that are commercially available today are not sufficiently frame-resistant for interior paneling of airplanes because they have an increased (impermissible) fire load, especially smoke density. In the field of electronics, halogen-substituted epoxide resins are known that have high flame-resistance. However, in the case of fire the presence of halogens leads to the generation of highly toxic and highly corrosive gases so that the use of halogen-substituted epoxide resins is not possible.
Cyanate resins instead exhibit already an intrinsic flame resistance because of their crosslinked structure (as a result of the high nitrogen contents). They combine a low heat release rate with a minimal smoke density and a low proportion of toxic gases in a fire situation.
In the literature there are a few proposals for producing prepreg materials based on cyanate resins. For example, the Japanese abstract published under publication No. 2002-194212 A discloses a curable resin composition for laminates or prepregs therefor that comprises a cyanate ester, a monofunctional phenol component, a polyphenylene ether resin, a flame retardant that cannot react with the cyanate ester, as well as a metal-containing reaction catalyst. The heat-resistant shapeable resin is used to produce a prepreg and a laminate. The prepreg is suitable for producing multi-layer printed circuits with very excellent dielectric properties. The Japanese abstract with publication No. 2002-146185 proposes a similar resin for the same type of application. However, instead of the polyphenylene ether resin a polyethylene resin is used. According to Japanese abstract with publication No. 02-302446 a prepreg and a printed circuit board produced therefrom are disclosed wherein the components for the impregnation resin is a polyaromatic cyanate, a multi-valent phenol, a polyaromatic cyanate phenol, a catalyst, and, as needed, a flame retardant. The resin composition disclosed in EP 0 889 096 A2 is also provided for use in connection with printed circuit boards; the resin composition is produced from a modified cyanate ester, a mono-functional phenol component, a polyphenylene ether resin, as well as a flame retardant. The use of multi-functional phenols is described in this printed publication as being unfavorable because the hydroxy group on one side of the molecule will not react and remain within the macromolecule; this is said to worsen the dielectric properties required for the application.
EP 0 295 375 A2 proposes to provide prepregs with a removable film that is coated with silicone in order to ensure a long-lasting tack. The resin of the prepregs is comprised of a cyanate-functionalized base material that contains additional components such as epoxy resins or maleimide resins.
Japanese abstract 03-243634 A discloses a resin that is comprised of 2-30 percent by weight of the reaction product of neopentyl glycol and terephthalic acid chloride, i.e., an oligo ester having an average molecular weight of 200-2,000 and hydroxy groups at both ends as well as 98-70 percent by weight of a resin of a cyanate ester component and a bis-maleimide component. Organic and inorganic fibers are impregnated with these resins.
The effect of multi-functional phenols on cross-linking of cyanate resins has already been studied in the past on a model system under theoretical, in particular, kinetic considerations.